The Phenomenon: Electromagnetic Coupling Between PLC and DSL
We consider the case where the following statements are both true:                A premise contains a point of termination for digital subscriber line (DSL), defined here as ITU G.993.1, ITU G.993.2, or a similar multi-tone-modulated protocol.        The same premise employs power line communications (PLC), defined here as HomePlug AV, ITU G.hn, or a similar protocol, whose implementation provides a means for shaping the transmit power spectral density (PSD).        
The physical medium for DSL is typically voice-grade telephone cable or data-grade cable such as Cat5. The physical medium for PLC is typically the electric power wiring of the premise.
Although DSL and PLC signals nominally travel on separate physical media that are not intentionally coupled, the media nonetheless are coupled electromagnetically to some extent. Field testing by the inventor and independent parties confirms what stands to reason: that the media couple to an extent determined by physical factors that differ from premise to premise and from location to location within the premise.
If DSL and PLC had disjoint spectra, then parasitic coupling between them might be inconsequential. However, the spectra of DSL and PLC are not disjoint. They overlap at 2 MHz and above. Testing confirms what stands to reason: that parasitic coupling is sufficient at some premises, at some locations, to degrade performance significantly.
We address the case of PLC parasitically coupling into DSL. This case is of more practical concern than the reverse case because of the following:                PLC has no analogue to DSL's time-consuming training phase. PLC essentially trains continually as it carries traffic.        PLC was designed to operate on a more-hostile medium than DSL: power line versus phone line.        PLC signal is time-division-multiplexed. PLC transmit duty cycle varies according to traffic and approaches zero when there is no traffic. DSL signal, by contrast, is frequency-division-duplexed. DSL transmit duty cycle in both directions is 100%, independent of traffic.        Therefore, it should be easier for PLC to adapt automatically to coupled DSL than the reverse.        Therefore, it seems more fruitful to adjust PLC to get it out of DSL's way than to adjust DSL to get it out of PLC's way.        
It stands to reason, and testing confirms, that:                The coupling is highly frequency-dependent. Some frequencies couple much more strongly than others.        The coupling is substantially linear versus power level. Scaling the transmitted power by β results in scaling the coupled power by β.        The coupling can be modeled as a frequency-dependent “transfer function” that specifies the ratio of coupled power at DSL receiver to output power at PLC transmitter.        There is a distinct transfer function from each power receptacle in the premise to each phone jack in the premise. Therefore, at each premise, each PLC transmitter has a distinct transfer function to the DSL receiver.        
It stands to reason that the transmissions of PLC could couple strongly enough into the DSL to cause disturbances such as retraining and uncorrectable bit errors. Retraining of the DSL is not guaranteed to be effective. It is not guaranteed to adapt DSL to PLC successfully, because PLC transmissions are sporadic, and PLC transmissions may enter a lull while the retraining is performed. Subsequent PLC transmissions might cause retraining again. Even if successful, retraining to accommodate PLC might yield an unacceptably low DSL bit rate.
Tests performed by independent parties show that coupling from PLC into DSL can reduce the quality of service of DSL to below the minimum level needed to provide acceptable performance for the intended application, such as IPTV. Parasitic coupling of PLC into DSL is a phenomenon of practical, and commercial consequence.